Dimethyl biphenyl (DMBP) compounds are useful intermediates in the production of a variety of commercially valuable products, including polyesters and plasticizers for PVC and other polymer compositions. DMBP compounds may be readily converted to an ester plasticizer by a process comprising oxidation of the DMBP to produce the corresponding mono- or dicarboxylic acid followed by esterification with a long chain alcohol.
For example, 4,4′-biphenyl-dicarboxylic acid, optionally together with 3,4′-biphenyl dicarboxylic acid, is a potential precursor, either alone or as a modifier for polyethylene terephthalate (PET), in the production of polyester fibers, engineering plastics, liquid crystal polymers for electronic and mechanical devices, and films with high heat resistance and strength.
Processes to produce DMBP compounds generally yield a mixture of all six DMBP isomers, namely 2,2′-, 2,3′-, 2,4′-, 3,3′-, 3,4′- and 4,4′-DMBP (see, for example, International Patent Application Publication No. WO 2015/112252).

Such processes also yield DMBP containing product streams comprising other non-DMBP components including one or more of cyclopentadienyl toluenes (CPDTs), 4-methylcyclohexyl toluenes (4,X′-MCHTs, where X=2, 3 or 4), 3-methylcyclohexyl toluenes (3,X′-MCHTs, where X=2, 3 or 4), 2-methylcyclohexyl toluenes (2,X′-MCHTSs, where X=2, 3 or 4), 1-methylcyclohexyl toluenes (1,X′-MCHTs, where X=2, 3 or 4), ethyl- or dimethyl-cyclopentyl toluenes (CPTs), and dimethyl bicyclohexanes (DMBCHs).
For certain applications, it is desirable to maximize the purity of individual DMBP isomers, particularly the 3,3′-, 3,4′- and 4,4′-DMBP isomers. However, in view of the proximity of the boiling points of the components in the DMBP product stream, separation based on distillation is challenging (see Table 1 below).
TABLE 1IsomerNormal Boiling Point (° C.)Fusion Temperature (° C.)1,X′-MCHT2702,2′-MCHT2692,3′-MCHT2692,4′-MCHT2753,2′-MCHT2713,3′-MCHT2703,4′-MCHT2714,2′-MCHT2724,3′-MCHT2714,4′-MCHT2732,2′-DMBP261192,3′-DMBP2722,4′-DMBP275−243,3′-DMBP28983,4′-DMBP293124,4′-DMBP296115
Based on boiling point differences it is possible to separate the DMBP product stream into a first stream comprising the 3,3′-, 3,4′- and 4,4′-DMBP isomers and a second stream comprising a mixture of the 2,X′-DMBP isomers, where X=2, 3 or 4, and the various MCHT isomers, utilizing, for example, fractional distillation.
However, it would also be desirable to separate the 2,X′-DMBP isomers (where X=2, 3 or 4) from the MCHT components so that the 2,X′-DMBP isomers may be converted into the more desirable 3,3′-, 3,4′- and 4,4′-DMBP isomers. However, in view of the very close boiling points of the 2,X′-DMBP isomers to those of the MCHT components this is challenging.
Furthermore, separation of the 3,3′-, 3,4′- and 4,4′-DMBP isomers from each other based on boiling point also presents a challenge, particularly separation of the 3,4′-isomer from the 4,4′-isomer which have very close boiling points. Based on heat of fusion differences it is in principle possible to effect separation of 3,3′-, 3,4′- and 4,4′-DMBP isomers via crystallization. However, because the relative proportions of some of these isomers in a mixture may be small, separation by crystallization may not be commercially attractive.
In view of the above it would be desirable to provide alternative processes for the production and separation of DMBP isomers, particularly processes that may improve the overall yield of the desirable 3,3′-, 3,4′- and 4,4′-DMBP isomers and also provide each of these isomers in an isomerically pure form.
It is known that certain adsorbents, for example zeolites, can be used to separate individual hydrocarbons from mixtures thereof. Adsorptive separation may be useful where the components to be separated have similar physical properties such as boiling point and melting points. For example, utilizing zeolites it is possible to selectively separate a predetermined xylene from a mixture of xylene isomers. See, for example, United States Patent Application Publication No. 2009/0326310 and references therein.
The reference in this specification to any prior publication (or information derived from it), or to any matter which is known, is not, and should not be taken as an acknowledgement or admission or any form of suggestion that the prior publication (or information derived from it) or known matter forms part of the common general knowledge in the field of endeavour to which this specification relates.